Prevent image thumbnails from being created during import

disable-thumbnail-generation.php

<?php
/**
 * Filtering 'intermediate_image_sizes_advanced' to return an empty array
 * prior to calling media_sideload_image() will prevent thumbnails from being created.
 */
add_filter( 'intermediate_image_sizes_advanced', '__return_empty_array' );
$attachment_id = media_sideload_image( $image_url, $post_id, '', 'id' );
remove_filter( 'intermediate_image_sizes_advanced', '__return_empty_array' );